The activity of auditory-nerve fibers will be recorded in anesthetized cats in response to electric stimuli applied through intracochlear electrodes similar to those used in humans. Such basic physiological information is likely to be of practical use in designing improved multiple-channel cochlear prostheses. A first series of experiments will test predictions of biophysical model of extracellular stimulation of myelinated nerve fibers developed in Project 4. These experiments will provide very fundamental information important for understanding stimulation not only of auditory-nerve fibers, but of myelinated neurons in general. A second series of experiments will study the nonsimultaneous electrode interactions which occur when current pulses are applied in rapid sequence through the same or different electrodes. These interactions, which demonstrate both masking and sensitization, are likely to play an important role in the continuous interleaved sampling speech processors developed at RTI. Results of these experiments will be compared with psychophysical and evoked-potential measurements of interactions in Projects 1 and 3. A third series of experiments will examine the temporal patterns of discharge elicited in auditory-nerve fibers by complex electric stimuli resembling those produced by the speech processors of existing devices. In particular, the neural activity produced by continuous-analog stimulation as in the Ineraid device will be compared with that produced with different continuous interleaved sampling processors. Characteristics of these processors will be manipulated in parallel with the study of new processing schemes in Project 1. These experiments are specifically directed towards improvements in speech-processor design.